This invention relates to MSK digital data transmission systems and more particularly to a MSK digital data synchronization detector for such systems.
MSK is an abbreviation for "minimum shift keying" and may be defined as a signal where the transmitted wave is a phase continuous frequency shift waveform. For each unit time interval the instantaneous frequency is a constant being either a high frequency or a low frequency relative to a center frequency. The high frequency is such as to advance by one phase revolution relative to the carrier frequency in four unit time intervals. The low frequency is such as to fall behind by one phase revolution relative to the carrier frequency in four unit time intervals.
MSK digital data synchronization detectors are used to determine the presence or absence of a MSK digital data signal and to synchronize in time and phase to such a signal. A MSK signal is hard to detect because there is no amplitude modulation or particular preferred phase that can be detected.
One prior art MSK digital data synchronization detector employs a method of detection which requires searching in various carrier phase and bit times looking for a stable tracking procedure. Stable tracking behavior and low orthogonal signals at the sampling time indicate the presence of a MSK digital data signal.
One of the disadvantages of the above-mentioned prior art detector is that it requires a higher signal-to-noise ratio or multiple search at a given signal-to-noise ratio to acquire synchronization with the same probability of acquisition and false alarm.